1. Field of the Invention
This invention relates to a screw tightening device which automatically tightens a screw to a predetermined torque, and more particularly, to a screw tightening device which has an electric motor for driving a tightening bit and in which the screw can be prevented from being tightened to an excessive torque due to inertia of the motor when the motor is stopped.
2. Description of the Prior Art
There has been known a screw tightening device comprising a DC motor which is connected to a bit by way of a coupling. In such a screw tightening device, the tightening torque is successively detected, and the motor is de-energized a predetermined time after the torque detected reaches a predetermined value. When the rotational speed of the motor is increased, for instance, up to 800 to 2000 rpm in order to improve the working efficiency, the rotational moment of the motor can be as large as 0.2 to 1 Kg/cm.sup.2, and the impact of the screw head on the surface of the workpiece increases to such an extent that it can damage the workpiece, particularly when the workpiece is of plastic resin. This problem may probably be solved by a torque clutch which is inserted between the output shaft of the motor and the coupling and releases the excessive torque. However, this approach is disadvantageous in that the screw tightening device becomes complicated in structure and large in size, and adjustment and maintenance of the device becomes troublesome.
Further, there has been known a screw tightening device in which a torsion spring is connected between the DC motor and the bit as the coupling so that the driving force of the motor is absorbed by the torsion spring when the screw head impacts against the workpiece. However, since the torsion spring has a small spring constant in order to effectively absorb the driving force, the torsion spring is largely twisted when the screw head impacts against the workpiece and the motor can be excessively reversed under the returning force of the spring when it is de-energized, which can loosen the screw.